In recent years, in order to purify environmental pollutants contained in an exhaust gas from an engine, an exhaust gas purifier system having a diesel particulate filter (DPF) unit, a lean NOx trap (LNT) unit, and the like provided in an exhaust pipe has been developed. In such an exhaust gas purifier system, the unburned fuel is added to the exhaust gas, and the exhaust gas is heated by oxidizing and burning the fuel using an oxidation catalyst, so that the purifier is regenerated using the exhaust gas having a high temperature.
As a means for adding the unburned fuel to the exhaust gas, exhaust pipe injection attracts interest, in which exhaust gas recirculation (EGR) control can be applied even when a fuel is added, and a fuel consumption necessary to increase a temperature is suppressed to a low value without generating oil dilution in an engine cylinder. The exhaust pipe injection is a method of adding unburned fuel to the exhaust gas from the exhaust pipe injector provided in the exhaust pipe (for example, refer to Patent Documents 1 and 2)
FIG. 4 illustrates a configuration of the exhaust pipe injection system in the related art.
The exhaust-pipe injection system 41 that performs exhaust pipe injection includes a supply pump 12 for pressurizing and sending a fuel, a fuel filter 13 for removing impurities mixed in the fuel, a common-rail injection system 14 that accumulates the pressurized fuel, an in-cylinder injector 15 that injects the fuel accumulated in the common-rail injection system 14 into the inside of the engine cylinder, and an exhaust pipe injector 16 that injects the fuel obtained by removing impurities using the fuel filter 13 into the exhaust pipe.
In the exhaust-pipe injection system 41, the fuel supplied from the fuel tank to the supply pump 12 via a supply line 17 is primarily pressurized and is sent by the supply pump 12, so that the pressurized fuel passes through a primary pressurizing line 18 and the fuel filter 13 provided in the primary pressurizing line 18 and flows to the supply pump 12 again.
The supply pump 12 is driven by a crankshaft of an engine.
The fuel flowing to the supply pump 12 again is secondarily pressurized and is sent by the supply pump 12. The pressurized fuel passes through the secondary pressurizing line 19 and is accumulated in the common-rail injection system 14.
The high-pressure fuel accumulated in the common-rail injection system 14 is injected into the inside of the engine cylinder from the in-cylinder injector 15 via a fuel injection line 20 and is consumed for an engine combustion operation. A surplus fuel unconsumed in the in-cylinder injector 15 and a surplus fuel in the common-rail injection system 14 are returned from return lines 21 and 22 and are discharged to the fuel tank from a discharge line 23 connected to the return lines 21 and 22.
A part of the fuel passing through the fuel filter 13 is split to a fuel line 24 connected to a downstream side of the primary pressurizing line 18 and is supplied to the exhaust pipe injector 16 via the fuel line 24. In the fuel filter 13, a fuel pressure sensor 25 for monitoring an injection pressure in exhaust pipe injection is provided in an upstream side from a filter unit (not illustrated) in the fuel filter 13.
The in-cylinder injector 15, the exhaust pipe injector 16, and the fuel pressure sensor 25 are connected to an electronic control unit (ECU) 26. The ECU 26 controls the in-cylinder injector 15. The ECU 26 includes an injection control device 27 that adjusts an exhaust injection amount of the fuel by controlling an open/close operation of the exhaust pipe injector 16 based on a monitoring value of the fuel pressure input from the fuel pressure sensor 25 and a desired injection amount.
In this exhaust-pipe injection system 41, exhaust pipe injection is performed by driving the exhaust pipe injector 16 based on a duty ratio. The duty ratio refers to a ratio of an open time (injection time) of the exhaust pipe injector 16 per unit time. The injection amount (exhaust injection amount) from the exhaust pipe injector 16 is adjusted by controlling the duty ratio of the exhaust pipe injector 16.
When the fuel is injected from the exhaust pipe injector 16 to the exhaust pipe, an internal pressure of the fuel line 24 tends to temporarily decrease so as to reduce the exhaust injection amount. This is caused by a balance between a fuel supply of the supply pump 12 and a fuel reduction caused by the injection. In particular, this tendency becomes significant when an engine revolution is low, and an in-cylinder injection amount is large (low revolution and high torque) because the fuel supply amount of the supply pump 12 is small, and the fuel injection amount of the engine is large. Therefore, it is necessary to monitor the injection pressure of exhaust pipe injection (fuel pressure in the fuel line 24) and correct a reduction of the injection amount caused by the lowered pressure by increasing the duty ratio.
Typically, the exhaust injection amount is controlled using a 3-dimensional map (typically, called a BP map) in which an injection characteristic of the exhaust pipe injector 16 is expressed using a duty ratio for the injection pressure and the injection amount (that is, the exhaust injection amount is controlled by determining the duty ratio of the exhaust pipe injector 16 with reference to the BP map using the injection pressure and the desired injection amount). It is desirable that the injection pressure is measured by arranging the fuel pressure sensor in an exhaust injection path as close to the exhaust pipe injector 16 as possible. This is because a pressure fluctuation in the vicinity of the exhaust pipe injector 16 affects a change of the exhaust injection amount.